John T. M. Lyles wrote:
>Unfortunately, the old microwave oven test doesn't work well enough to
>apply the results, as constant fields are not produced, they purposely
>move them around to cook a chicken uniformly. Applying a fixed electric
>field in one direction through a thick dielectric, with minimal spaces
>(air gaps) between it and the metal, is the exact method used to get
>temperature rise in dielectric heating industry.
I think he microwave oven test does provide useful information - within
its limitations, of course. If you test similar samples of various
materials at the same time (relying on the turntable or mode-stirring to
subject them to similar average power densities) then you can tell by
touch which material is better or worse than the next one.
Loss tangents will be lower at lower frequencies, but (for the types of
materials that you'd ever consider as RF insulators) the ranking order -
from best to worst of the samples tested - is very unlikely to change.
What the microwave oven test will NOT tell you is whether a given
material will be OK in your particular application. It all depends on
the localized RF field strength, which can be very difficult to predict,
or even to visualize in your mind's eye.
There are examples where materials with a 'poor' reputation are
perfectly OK, because the RF field strength where they're being used is
actually quite low... but if we're dumb enough, it is possible to burn
out even a 'superb' material like Teflon.
In the end, it's probably better to play safe like John does. By all
means use materials with good RF properties (and IMO the microwave oven
test can be helpful in selecting plastics) and use the minimum possible
thickness to avoid internal heating.
There seems no reason to make a chimney out of thick material. The only
reason to use thick-walled tubing would be if you think it might begin
to soften - which says it's the wrong material anyway. Materials that
have worked for me in VHF/UHF amps include rolled Teflon sheet or mylar
sheet, with the vertical seam secured by thin mylar parcel tape; and
various kinds of glass tubing, glued to the metalwork with silicone
sealant. Like John, I wouldn't willingly use PVC... it might be OK in
your particular application, but why risk it when better materials are
easily available?
Will's fish paper might be fine, too, in many situations. (Worst case,
it would leave only a small amount of ash :-)
--
73 from Ian GM3SEK
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